Harden bad placeholder checks on statics/consts #97508
                
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Resubmission of #89161
Fixes #88643
In #83739, I added a check for trait objects on statics/consts but it wasn't robust.
is_suggestable_infer_tyfn does a more strict check and finds more bad placeholders. See #89161 (comment) for the more detailed explanation.r? @pnkfelix as you're the reviewer of the previous PR